Method and apparatus for programming liquid scintillation spectrometers

ABSTRACT

In an automatic scintillation counting device adapted to detect and measure the radioactivity of a multiplicity of samples supplied in trays or other sets, provisions are made for automatically changing one or more of the analog counting parameters, or analog-plus-digital parameters, for each set. The changes are effected automatically as each sample set comes into counting position, thereby permitting unattended automatic counting of samples requiring entirely different counting parameters. At the analyst&#39;&#39;s option, two or more sets may be counted under conditions established for one of the sets. Provisions are also made for retaining the operation of the master, or front panel, control when it is unnecessary to vary the counting program parameters for particular sets.

United States Patent 1191 Toman et al.

[ Feb. 12, 1974 METHOD AND APPARATUS FOR 3,604,935 9/1971 Nather 250/106 sc PROGRAMMING LIQUID SCINTILLATION 3,560,744 2/1971 Jordan 250/106 SC SPECTROMETERS E J w L Primary xamine rames awrence [75] Inventors: Joseph R. Toman, Naperville;

Edmund Frank, C g both of m Assistant ExammerB. C. Anderson [73] Assignee: Packard Instrument Company, Inc., [57] ABSTRACT Downers Grove, Ill. In an automatic scintillation counting device adapted [22] Fled: 1971 to detect and measure the radioactivity of a multiplic- [21] A 1 N 133,359 ity of samples supplied in trays or other sets, provisions are made for automatically changing one or more of the analo countin arameters, or analo plus-digital parame ers, for e ac h set. The changes afe [51] Int. Cl. G21}! 5/00 effected automatically as each sample Set comes into [58] Fleld of Search 250/106 SC, 71.5 R Counting position, thereby permitting unattended tomatic counting of samples requiring entirely differ- [56] Reerences C'ted ent counting parameters. At the analysts option, two

UNITED STATES PATENTS or more sets may be counted under conditions estab- 3,188,468 6/1965 Packard 250/106 sc lished r n f h sets. Provisions are also made for 3,437,812 4/1969 Packard 250/106 SC retaining the operation of the master, or front panel,

3,499, 3/1970 Cavanaugh, r-

.. 250/106 C control when it is unnecessary to vary the counting 3,271,574 9/1966 Dawson et a1. 250/1 6 SC program parameters for particular sets. T

3,246,156 4/1966 Frank et al 250/106 SC 3,571,596 3/1971 Frank 250/106 SC 9 Claims, 18 Drawing; Figures ANALOG DISPLAY AND DIGITAL /7a 7 Y CONTROL Y AUTO; 510. 1 A47 A4! zda L 22 zaf OFF-\ CHANNEL I 0099457] (R) a; af/lxxry/wAv/az/lz gz gf 525i? m? COUNT [GREECE]- u (R) 1 zg orelsmmlrmslsnossl cm IRATE Z04 i 2 1. L LEVEL R) R G GAIN(R) GAIN(G) 94! (G) START. RUE PW5R Low LEVEL PRESEI' PRESET couur CYCLES 107mm u-- 10pm Wm Reg 4x221) commence VIZ *4; n

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PATENTEBFEBI 219M 1 0 MW? a WWW W M; 1 a m Ma W7 PATENTEUFEB 1 21974 SHEET USUF 14 M92276 SHEET UBOF l4 .4rrazA/zra x J 4 1 w llll L WWW 4 4 fix FM MM K Z Z [a m,

W M AOM I l l l I l IIL w. fi m PATENIEBFEBI 21914 PMENTEU FEB l 2 I974 SHEET IUOF 14 METHOD AND APPARATUS FOR PROGRAMMING LIQUID SCINTILLATION SPECTROMETERS CROSS REFERENCE TO RELATED APPLICATIONS AND PATENTS Packard U.S. Pat. No. 3,] l4,835

Packard U.S. Pat; No. 3,I88,468

Cavanaugh U.S. Pat. No. 3,499,149

Bristol Ser. No. 629,462 filed Apr. l0, I967 Frank Ser. No. 27,405 filed Apr. 10, I970 Frank et al. Ser. No 27,406 filed Apr. 10, I970 Frank Ser. No. 27,411 10,

SUMMARY AND BACKGROUND auxiliaries it is now possible to place a batch of samples into a spectrometer apparatus and have the apparatus function unattended to detect and measure the radioactivity, correct for inherent errors, and print out the results on a computer-print-out sheet or on magnetically or mechanically-readable records.

There is, however, one limitation of many such automatic spectrometers. Customarily a laboratory will have one spectrometer which is used by a number of researchers or research teams. Each researcher generates a set or series of samples which, for optimum scintillator accuracy, may require a particular set of predetermined counting program parameters or conditions. For example, different radionuclides may be used, the activity levels may differ by orders of magnitude, or two or more radionuclides may be present. It is customary in such circumstances for each researcher to alter the various counting condition controls for his or her set of samples, and then either restore or re-set the controls for the next set. This necessarily creates the possibility of human error by incorrectly adjusting the various function controls and, from' an inconvenience standpoint, prevents unattended overnight or overweekend automatic counting.

Accordingly, an object of the invention is to provide a simple, fully automatic, system for changing the scintillator counting program parameters either analog or analog-plus-digital for each set of samples. A related object is to provide replacement program-control modules so that each researcher can adjust the modules controls once for his or her project, and thereafter be assured that the same program parameters are being used for counting all the projects samples. An overall object is to provide an automatic scintillation counting device, including automatic sample handling and transferring equipment, capable of changing one or more or all counting program parameters for each set of samples, and in whichthe changing is effected in response to the particular set of samples being (individually) counted.

Other and further objects include: provisions for counting two or more sets under program counting conditions established for one of the sets; for retaining the standard master, or front panel, controls when it is unnecessary to vary the programs for particular sample sets; and for utilizing various improved subcomponents for the system.

In the ensuing description, the system of the invention is applied to an exemplary liquid scintillation spectrometer, including sample handling and transferring equipment, for automatically processing a multiplicity of radioactive samples. The samples are transferred in sets to a sample counting zone for individual counting. It will be appreciated, however, that this type of scintillator and transfer equipment is exemplary only, and that the sets need not be defined by discrete trays or magazines, but may simply be regions or portions of a single magazine or of a belt-fed sample transferring mechanism, all of which are well known in the art.

BRIEF-DESCRIPTION OF DRAWINGS Other objects and advantages of the invention will become apparent as the following description proceeds, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a fragmentary overall perspective of a liquid scintillation spectrometer embodying the invention, including front panel and program controls;

FIG. 2 is a front elevation of the front panel on the spectrometer of FIG. 1; 7

FIG. 3 is a top plan view showing the overall arrangement of a sample changing apparatus and associated detection and counting mechanisms, as employed in the spectrometer of FIG. 1;

FIG. 4 is a perspective showing, in somewhat stylized or schematic form, certain of the electrical components, wiring, and connections on. the sample changing apparatus of FIG. 3;

FIG. 5 is a diagrammatic block-and-Iine representation of an illustrative prior art multiple channel scintillation spectrometer capable of embodying the present invention;

FIG. 6 is a diagrammatic, stylized, block-and-line representation of a programming system of the invention, including optional provision for programmed digital control;

FIG. 7 is a diagrammatic, more detailed, block-andline representation of the pulse height analyzer (P.H.A.) component of the spectrometer of FIG. 5, illustrating certain of the connector points associated with the invention;

FIGS. 80, 8b, and 8c, in conjunction, are a diagrammatic detailed block-and-line representation of the tray program control circuitry of the invention;

FIG. 9 is a wiring diagram of a single tray program ming card for interconnection into the control circuitry of FIG. 8;

FIG. 10 is a wiring diagram of the dc. attenuator and amplifier (D.C.ATTN & AMP) component of the spectrometer of FIG. 5;

FIG. 11 is a perspective of an alternative embodiment of the invention, with provisions for both analog and digital program selection of the spectrometer;

FIG. 12 is a front view of an illustrative program card for the alternative embodiment of FIG. 11.

FIG. 13 is a diagrammatic partial schematic representation of a modified FIG. 8 circuit for use when it is desired to program several trays from a single tray programming card;

FIG. 14 is a diagrammatic partial schematic representation of a dummy tray programming card for use with the circuit of FIG. 13;

FIG. 15 is a schematic block and-line representation of one form of system for providing digital as well as analog program control; and

FIG. 16 is a similarschematic of an alternative form of system for digital and analog control.

While the invention is susceptible of various modifications and alternative forms, a specific embodiment thereof has been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular form disclosed, but, on the contrary, the intention is to cover all modifications.

equivalents and alternatives falling within the spirit and scope of the invention as expressed in the appended claims.

THE ENVIRONMENT OF THE INVENTION Before treating the present invention in detail, it will be helpful first to consider briefly the prior art background or environment. In radioactivity measurements, it is the most frequent objective to determine the rate at which decay events in an isotope present in a radioactive source occur, this rate generally being expressed as counts per unit time, e.g., counts per minute. The quantity of a particular isotope present in a radioactive source is in general proportional to the rate of decay events produced by that isotope, such rate being termed the activity level of the source. As a generalization, the decay events or radiation emanations from a radioactive source are, for purposes of measurement or counting, converted into-corresponding voltage or current pulses which can then be counted. The pulses may be counted for a predetermined time, or a predetermined number of pulses may be counted, the ratio of counted pulses to the elapsed time period being indicative of the activity level. In some instances the voltage pulses may be fed to a direct-reading or recording rate meter which indicates the activity level.

GENERAL ORGANIZATION AND OPERATION OF SPECTROMETER The spectrometer illustrated diagrammatically at 101 in FIG. 1 includes a radioactive source 132 (FIGS. 3, 5), disposed in operative relationship to one or two light-responsive detectors or proportional transducers 124, 125 (FIGS. 3, 5). The transducers serve to convert radiation from decay events within the source 132 into corresponding electrical signals, e.g., voltage pulses, the transducer may be any one of a variety available in the art, such for example as a photomultiplier tube observing a sodium iodide (thalium activated) crystal which converts radioactive decay events from gammaemitting isotopes into light flashes. Regardless of the particular type of detector and transducer employed, it should have a proportional characteristic, i.e., each voltage pulse will be substantially proportional to the energy of the decay event which produces it.

The voltage pulses so derived by the detector or transducers 124, 125 are, however, of relatively low amplitude. It is impractical, if not impossible, to discriminate these pulses on the basis of differences in their amplitudes and to count or measure their rate of occurrence. Accordingly, and with reference to FIG. 5, the pulses from the transducers 124, 125 (FIGS. 3, 5)

are passed through one or more channels, each comprising a linear amplifier 127 and a pulse height analyzer (P.H.A.) 129. Because the pulses received by the analyzer 129 have been amplified and thus occupy a fairly wide spectrum of amplitudes, the analyzer selects only those pulses which lie above or below certain preselected amplitudes, and passes them to a final indicat ing device such as a rate meter or a scaler, the latter being shown at 157 (FIG. 2). By adjusting the amplitude band or window of pulse amplitudes passed by the analyzer 129, the background pulses (resulting either from spurious radiation, cosmic rays, or noise in the transducer) is substantially reduced, so that the count received by the scaler is comprised principally of pulses resulting from the activity of the source being measured. The background pulses to a large extent have amplitudes that fall outside the acceptance band of the analyzer after the latter has been adjusted. The background count passed by the analyzer may, of course, be measured for a given time interval with no radioactive source present, and then subtracted from subsequent scaler counts received with radioactive sources present.

Because the pulses applied to the input of the analyzer 129 have amplitudes substantially proportional to the energy of corresponding decay events in the source 132, the analyzer may be adjusted successively to pass bands of pulse amplitudes, and the energy spectrum of the decay events in an isotope thus plotted. Once that spectrum has been measured, or is known, the analyzer 129 may be set to pass an amplitude band or window in which a fairly large proportion of all pulses derived from the isotope decay events are passed and counted, yet in which the background or spurious pulses passed to the scaler are relatively few.

LIQUID SCINTILLATION AND COINCIDENCE MONITORING In spectrometry involving isotopes which produce radiation particles having relatively low penetrating power, namely alpha and beta particles, and particularly with samples or sources of low activity levels, the detector or tranducer preferably includes a solution of liquid scintillator into which the radioactive substance (sample) is added. Light flashes in this solution, resulting from decay events in the isotope, are transmitted to a photosensitive electrical device, preferably a photomultiplier tube. Because such photomultiplier tubes are to an undesirable degree prone to produce spurious voltage pulses due to dark noise current which is primarily caused by thermally induced electron emission, it has been common practice to employ coincidence monitoring in order to preclude counting of these spurious voltage pulses. Such a liquid scintillation spectrometer with coincidence monitoring, and intended primarily for work with alpha and beta radiation isotopes, is diagrammatically illustrated in FIG. 5.

Referring to FIG. 5, the radioactive source is shown at 132 as a sample of isotope-containing substance dissolved or suspended in a liquid scintillator, the latter being in a container having light-transmissive walls. Aromatic ethers are commonly employed as solvents, although numerous other solvents are known in the art. Any one of numerous commercially obtainable scintillators or fluorescent materials is also dissolved in the solvent for the purpose of converting the radiant energy resulting from a decay event (for example, an

alpha or beta decay event) intolight energy. Finally, the sample includes the radioactive material, which is or contains a radionuclide or isotope to be measured. The emission energy spectra of the different radionu clides may vary greatly, each having a characteristic knownspectrum. Such spectral characteristics are fully described in the scientific literature.

Where the maximum beta energy of the radionuclide is relatively low, typified by tritium (maximum beta energy of 0.018 mev.), carbon-l4 (0.15 mev.), and up to phosphorus-32 (1.71 mev.) or so, the light flashes in the scintillator solution are relatively weak, although proportional in intensity tothe energy of the decay events which produce them. It is for this reason that a very sensitive lightto-voltage transducer, such as a relatively high gain photomultiplier tube, is employed, and coincidence monitoring is utilized to reduce the effects of noise pulses therein.

Referring more specifically to FIG. 5, the spectrometer there illustrated includes a pair of photomultipliers as transducers 124, 125 which are placed contiguous with, or adjacent to, the sample 132 and energized from a variable high voltage source 144. The photomultipliers 124 and 125 serve the respective functions of providing pulses to be analyzed and pulses to monitor or gate the first pulses when both photomultipliers simultaneously respond. The outputs of photomultipliers 124,125 may, if desired, be preamplified through preamplifiers, not shown, if the output from the respective photomultipliers is insufficient, but with present day (1971 high gain photomultipliers such preamplifiers may frequently be omitted.

In order to reduce the number of thermal noise pulses in the photomultipliers, the sample 132, the photomultipliers 124,125, and the optional preamplifiers are advantageously all located within a cooled chamber or freezer, diagrammatically illustrated at 102 (FIG. 1

The outputs of the photomultipliers 124,125 or of their associated preamplifiers, are coupled to one or more channels, each composed of an (optional) attenuation unit and amplifier 127, a pulse height analyzer 129, and a logic circuit 135, one channel for each radionuclide in the sample. The photomultiplier output pulses are thus amplified to a predetermined extent, and passed to the pulse height analyzers 129, each of which provides a discriminated input signal for a suitable logic circuit 135 which, for example, may simply be an AND gate, which feeds into a control logic, scaler (scaler 157 onFIG. 2), and readout system 158 (on FIG. 5). Additional channels may be provided to accommodate additional channels for simultaneously counting a sample containing more than two different radionuclides, as explained below.

When a decay event (for example, a beta emission) occurs in the sample 132, a light scintillation is pro duced that is simultaneously detected by both photomultipliers l24,l25. Correspondingly, electrical signal pulses proportional in amplitude to the energy of the decay event (i.e., the amount of light observed by the respective photomultipliers) are fed to one or more of the pre-selected channels (Channel I, II," or III in FIG. 5), and amplified by the amplifiers 127 in that selected channel. These pulses are then analyzed in pulse height analyzer 129 which passes only a selected band of pulses. The analyzers 129 preferably are constructed and adjusted to pass all received pulses which exceed a predetermined low amplitude, that is, to serve as lower level discriminators (L.lL. DISC." in FIG. 7) and may (but need not) restrict the upper amplitude of passed pulses, i.e., as upper level discriminators (U.L. DISC.").

As will be explained below in connection with coincident counting, only when the analyzer 129 and the coincidence circuit 136 provides coincident, or simultaneous, input pulses to the AND gate in the logic circuit does the latter produce an output pulse which is controlled by the system 158 (i.e., scaler 159 in FIG. 2). If, on the other hand, coincident inputs are not present at the ANDgate, any pulse from the pulse height analyzer 129 is blocked from, and therefore not counted by, the scaler system. By way of example, when a thermal pulse is generated in either one of the photomultipliers, coincident input pulses will not be rec'eived by the coincident circuit 136 and presented at the AND gate of logic circuit 135, and hence no count will be recorded by the scaler 157 (FIG. 2). This is so even if the thermal pulse is within the amplitude band or window being passed by the analyzer.

MULTIPLE LABELED MEASUREMENTS AND MULTIPLE CHANNEL CIRCUIT As further shown in FIG. 5, the liquid scintillation and coincidence monitoring circuit may be adapted to detect, isolate, and measure the radioactivity emanating from a plurality of radionuclides within a single sample, the operations associated with each nuclide being performed simultaneously. The circuit of FIG. 5, for this purpose, 'includes one or more additional counting channels ?II and III (FIG. 5), each composed of an attenuator and amplifier unit 127, pulse height analyzer 129, logic circuit 135 (connected to the pulse height analyzer 129), and a scaler system 158, and if desired, even more such channels. Multiple channel units-are adapted for the independent measurement of the activity levels of two or more isotopes which are simultaneously present in a single source or sample, or which are present individually in mixed sources or samples; circuits and techniques for such detection and measurement are described, for examle, in Packard 835.

Very briefly, in multiple channel radioactivity detecting and measuring spectrometers, the output from the photomultipliers 124,125 (FIG. 5) is fed simultaneously to a plurality of channels, each containing an attenuation unit, an amplifier, a pulse height analyzer, and associated logic and scaler apparatus. In each channel the attenuation unit and/or amplifier amplify the pulse by an amount or factor which is different from that in the other channel or channels. If the spectral peaks of the respective radionuclides are separable, counting and monitoring of pulses from one radionuclide may be effected independent of pulses contributed by the other or others; if the spectral peaks overlap, computation may be required to determine the activity attributed to each radionuclide. Multiple channel circuits thus are capable of simultaneously measuring the activity levels of each of several different radionuclides provided each radionuclide has a different characteristic energy spectrum.

SUMMATION-TYPE MEASUREMENT CIRCUIT Although in some scintillation circuits only one of the photomultipliers 124,125 is used for detecting scintillations (the other serving only to exclude signals from the first photomultiplier that do not correspond to a scintil- Iation), a more efficient coincident counting circuit, generally termed a summation-type circuit (as shown in FIG. allows both photomultipliers 124,125 to observe the sample and to deliver a usable output signal corresponding to each decay event. In effect, a summation-type circuit adds algebraically the simultaneous output pulses of the two photomultiplier tubes 124,125 so that the input to the attenuation units and amplifiers is double the amplitude that can otherwise be obtained. Advantages of the summation circuitry are set forth in Bristol Ser. No. 629,462.

As shown in FIG. 5, the summation circuit utilizes the photomultipliers 124,125 positioned to observe the sample 132 as in the arrangement of FIG. 3. Again, preamplifiers may be used, and the assembly of sample, photomultipliers, and preamplifiers may be positioned in a refrigerated zone, not shown. Output pulses delivered by the photomultipliers 124,125 are both fed to a coincidence circuit 136 and to a summation circuit 137. The coincidence circuit 136 may typically comprise an amplifier and a pulse forming network associated with each line from one of the photomultipliers, with the pulse forming networks feeding into an AND gate which delivers an output signal only when signals from the photomultipliers 124,125 are simultaneous. The summation circuit 137 illustratively comprises a pair of preamplifiers and a summing amplifier to sum the signal pulses received from the photomultipliers. Thus, coincident pulses from the photomultipliers are effectively doubled in amplitude by the summation circuit 137 while non-coincident pulses corresponding to thermal noise in one of the photomultipliers remain at the initial level.

Further in the circuit of FIG. 5, the output signals from the summarion circuit 137 are fed to the pulse height analysis channels, of which three such channels are shown in the drawing. Each such channel comprises an attenuation unit and amplifier 127, a pulse height analyzer 129, and a logic circuit 135. Thus, all of the pulses from the summation circuit 137 are received by each pulse height analysis channel, although the only pulses transmitted from the corresponding logic unit will be those coincident pulses of proper amplitude which pass through the window established by the pulse height analyzer for that pulse height analysis channel.

Coincidence counting, that is, the rejection of noncoincident pulses caused by thermal noise in the photomultipliers or in their associated preamplifiers, is obtained by feeding the output from the coincidence circuit 136 to the logic circuit 135 associated with each pulse height analysis channel. Thus, only those signals from photomultipliers 124,125 which are generated simultaneously will open the AND gate in the logic circuit 135 and permit registration of the signal'by the sealers.

As indicated in FIG. 5, two or more pulse height analysis channels may be utilized to count simultaneously a sample containing two or more radionuclides, the method of using the summation-type circuit of FIG. 5 corresponding generally to that more fully set forth in Packard 835.

TYPICAL PHOTOMULTIPLIER ARRANGEMENT The structure and operation of typical photomultipliers and their associated circuitry have been fully described, as for example in Packard 835 and in Bristol Ser. No. 629,462, and accordingly their principles are well understood. However, to provide a background for an understanding of the circuitry constituting the present invention, a brief descritpion of photomultipliers and their operation will be presented.

When a decay event, for example, a beta emission from a radionuclide, occurs in the sample (132 of FIG. 5) it produces a light scintillation which is simultaneously detected by a photosensitive cathode, in one or both photomultiplier tubes 124,125. As the light rays impinge upon the cathode, electrons proportional in number to the energy of the light are emitted. The emitted electrons are attracted to the first dynode in the photomultiplier, which is at a higher voltage with respect to the cathode, thus producing by bombardment still more electrons which are in turn attracted to the next higher potential dynode. This process continues until the electrons reach the final element in the photomultiplier, the anode, the thus cause current flow through, and voltage pulses across, an output load resistance (not shown). These voltage pulses are sent to an output transformer (not shown) and appear at the input to the attenuation units and amplifiers of FIG. 5.

In order to simplify the ensuing discussion, isotope decay events will be referred to by way of example as beta decay events or beta emissions. It will be understood, however, that alpha and gamma radiation may be considered in the same way, even though, particularly in the case of gamma radiation, there may be mono-energetic spectra involved in some instances.

SPECTRAL DISTRIBUTIONS AND OPTIMUM COUNTING CONDITIONS It is well known that beta-emitting isotopes produce decay events which individually involve energies spread over a fairly wide range or spectrum. Each isotope has its own characteristic spectrum with a known maximum energy. A small proportion of the decay events have relatively high and low energies, while the majority have energies in the middle region between the upper and lower limits, the distribution being skewed rather markedly toward the lower limit. For a given gain of the transducer which forms a source of voltage or current pulses proportionally related to the energies of the decay events, the amplitude spectrum of the pulses corresponds to the energy spectrum of the decay events.

In the following discussion it is assumed, except as otherwise stated for explanatory or elaborational purposes, that the sample contains only a single radionuclide species. For a parallel discussion involving multiple-labeled samples, reference may be made to Packard 835.

If there were no spurious counts caused by thermal noise or background radiation, and if there were no quench effects (to be described presently), detecting all of the counts from a sample over a predetermined time period would provide a reading of sample activity in absolute units of counts per unit time. In practice however this is infeasible for a number of reasons, most important of which is the presence of low level background noise and high level background radiation, both of which generate spurious pulses that are indistinguishable from the actual decay-produced pulses, particularly at low and high levels of pulse height.

The effects of high and low level noise are customarily minimized by employing amplitude discriminators as pulse height analyzers to reject all pulses below a predetermined minimum amplitude and all pulses above a second, higher, predetermined amplitude. Thus, in FIG. 5, amplitude discriminators in the pulse height analyzers (e. g., 129) reject all pulses having an amplitude below a predetermined minimum and all pulses having an amplitude above a predetermined maximum. The region of accepted pulses is termed the window, and it will be evident that the width of the window determines the fraction of the total spectrum that is counted by the radiation detecting and measuring apparatus.

A given spectrometer may be provided with two or more windows, particularly when used for counting and measuring multiple-labeled samples. Additionally, and for reasons which will appear below, the spectrometer may have an external standardization circuit 156 (FIG. 5) including one or more integral channels, that is, amplification channels such as those including G-toinfinity (G. DISC. 165 on FIG. 5) and H-to-infinity (*H. DISC." on FIG. 5) discriminators. The effective window is from a predetermined amplitude G or H, respectively, and all pulses above that level are transmitted. Such integral windows are especially valuable in connection with the use of external standards to compensate for quenching effects.

QUENCl-IING AND QUENCHING COMPENSATION As suggested by the discussion of the counting window, the coungs per minute indicated by a scaler (e.g., scaler 157 of FIG. 2) will be substantially less than the activity of the sample expressed as disintegrations per minute. The proportionality factor relating sample activity (in disintegrations per minute) to the measured counts per minute is termed the counting efficiency, and is invariably less than 100 percent. Counting efficiencies of less than 100 percent are due mainly to three factors: limitations of equipment, the deliberate exclusion of a portion of the count by adopting the window technique, and quench.

The last element, quench or quenching, is chiefly of two types. Chemical quenching results from the presence of ingredients in the sample and scintillator (132 of FIGS. 3,5) will interfere with the conversion of beta particles to light scintillations; organic compounds having a nitro group are particularly serious offenders in this respect. Color quenching is the attenuation or diminution of the brightness of a scintillation caused by colored or color-absorbing ingredients in the sample and scintillator.

The effect of either type of quenching is identical, and results in an apparent diminution of pulse height as measured by a photomultiplier or as transmitted.

through a pulse height analysis channel. Systems and techniques have been devised for compensating for quench effects, as will be explained subsequently.

INTERNAL STANDARD Perhaps the earliest technique of quench compensation is by the use of an internal standard, that is, the procedure of determining the activity of a sample in a scintillator, adding to the sample a known amount of the same radionuclide as a standard, re-determining the activity, and computing the unquenched activity of the original sample from the ratio of the difference in the two measurements and the expected difference based on the known activity of the standard. On the assumption that quenching will be constant for similar samples, the activity measured for all such samples can then be corrected to allow for quench and other factors which reduce counting efficiency.

EXTERNAL STANDARD STRAIGHT COUNT The technique of internal standardization is rather cumbersome and laborious; it is slow, the original sample is invariably contaminated by the standard, there is opportunity for technician error, and frequently different standards must be used with different samples where the samples have widely differing activities. To avoid these limitations, the technique of external stan' dardization has been developed, and for more complete description of external standardization reference may be made to Packard 468, Cavanaugh l49, or Bristol Ser. No. 629,462.

Briefly, for external standardization, a sample is first counted alone, a high energy gamma-emitting source producing scintillations by Compton interaction is placed near the sample, and the two re-counted together. By relating the actual change in measured counts to the expected change produced by the external standard (as determined by previously counting the external standard in the presence of an unquenched standard), the un-quneched activity of the sample can then be computed.

By one technique, known as the straight count method (AUTO STD. position of the knob 173 of FIG. 2), determining the change in counts produced only by the external standard at any predetermined window or infinity channel (e.g., the G-to-infinity channel, via the G.DISC. of FIG. 5) it is possible to compute the effect of quenching on the samplepA series of samples having the same known activity is made up, containing progressively increased amounts of an ingredient known to produce quenching. Advantageously, these samples contain the same scintillator, solvent, and radionuclide as the unknown" samples. These samples are then counted in the presence of the external standard and a counting efficiency curve is plotted. Then, using the same channel or channels that were employed in preparing the curve to count the pulses produced by the external standard in the presence of an unknown sample, the curve yields a numerical value for the counting efficiency. When this efficiency is divided into the measured activity of the unknown sample counted in the absence of the external standard, the quotient is the true activity of the sample in disintegrations per minute.

NET EXTERNAL STANDARD RATIO A further improvement of the technique of external standardization entails the use of two channels (CHANNEL RATIO position of the knob 173 of FIG. 2), rather than one, to determine the amount of quenching of radiation produced by the external standard. This technique, known as the net external standard ratio procedure, is particularly adaptable to the automatizing of external standard quench determina tions.

Again inviting attention to FIG. 4, and particularly to the external standardization circuit 156, the channels for determining quenching effects of radiation from the 

1. In a scintillation spectrometer for automatically and sequentially handling and counting a multiplicty of radioactive samples, including A. a sample counting zone, B. means for counting said samples individually in said counting zone, including
 1. means responsive to radioactivity decay events for producing electrical pulses having an amplitude related to the energy of corresponding decay events,
 2. an amplification-discrimination channel receiving said electrical pulses, said channel having a. an electronic amplifier for amplifying pulses from said event responsive means, and b. upper and lower level discriminators for passing only a selected amplitude band of amplified pulses therethrough, and
 3. means for counting discriminated amplified signals as a measure of the number of said decay events, C. a first analog parameter control for controlling at least one of (i) the gain of said electronic amplifier, (ii) said upper level discriminator and (iii) said lower level discriminator, and D. means for transferring said multiplicity of samples in a plurality of sets to said sample counting zone for individual counting of samples, the improvement whereby at least one of the pulse amplifying amplifier parameters is programmed to effect counting of samples within each set at at least one predetermined analog parameter different from parameters for other sets, which comprises: E. a replaceable analog parameter control having predetermined control settings and associated with an individual one of said plurality of sets, F. means for determining the particular one of said sets then having its samples transferred to said sample counting zone, and G. means responsive to said set determining means for disconnecting said first analog parameter control out of, and for connecting said replaceable analog parameter control into, said amplifier-discriminator channel.
 2. an amplification-discrimination channel receiving said electrical pulses, said channel having a. an electronic amplifier for amplifying pulses from said event responsive means, and b. upper and lower level discriminators for passing only a selected amplitude band of amplified pulses therethrough, and
 2. Spectrometer of claim 1 including a plurality of said amplification-discrimination channels, wherein said first analog parameter control and said replaceable analog parameter control control at least one analog parameter in each of said channels.
 3. Spectrometer of claim 1 wherein said replaceable analog parameter control controls said gain and both of said discriminators.
 3. means for counting discriminated amplified signals as a measure of the number of said decay events, C. a first analog parameter control for controlling at least one of (i) the gain of said electronic amplifier, (ii) said upper level discriminator and (iii) said lower level discriminator, and D. means for transferring said multiplicity of samples in a plurality of sets to said sample counting zone for individual counting of samples, the improvement whereby at least one of the pulse amplifying amplifier parameters is programmed to effect counting of samples within each set at at least one predetermined analog parameter different from parameters for other sets, which comprises: E. a replaceable analog parameter control having predetermined control settings and associated with an individual one of said plurality of sets, F. means for determining the particular one of said sets then having its samples transferred to said sample counting zone, and G. means responsive to said set determining means for disconnecting said first analog parameter control out of, and for connecting said replaceable analog parameter control into, said amplifier-discriminator channel.
 4. Spectrometer of claim 1 wherein said replaceable analog parameter control controls both said discriminators but not said gain.
 5. Spectrometer of claim 1 wherein said replaceable analog parameter control additionally controls gain at 100 percent of gain and at 10 percent of gain.
 6. Spectrometer of claim 1 including at least one additional replaceable analog parameter control associated with one of said sets and adapted to select the replaceable analog parameter control associated with another of said sets.
 7. Spectrometer of claim 1 including manual switch means on said replaceable analog parameter control for retaining analog parameter controls by said first analog parameter control.
 8. Spectrometer of claim 1 including a separate replaceable analog parameter control associated with each one of said sets.
 9. Spectrometer of claim 1 including a first digital parameter control for controlling at least one digital parameter of said sample counting means, a replaceable digital parameter control having predetermined control settings and associated with said replaceable analog parameter control, and means responsive to said connecting or disconnecting of said replaceable analog parameter control for disconnecting said first digital parameter control out of, and for connecting said replaceable digital parameter control into, said sample counting means. 